The invention relates to a two-stroke engine as used, especially, as a drive engine in a portable handheld work apparatus such as a motor-driven chain saw, brushcutter, cutoff machine, blower apparatus or the like.
Mixture-scavenged two-stroke engines are used especially in portable handheld work apparatus because of the low weight and high power realization. In order to improve the exhaust-gas performance of mixture-scavenged two-stroke engines, German patent publication 2,650,834 discloses, in the context of a four-stroke engine, supplying essentially fuel-free gas (preferably air) via the transfer channels disposed close to the outlet and to introduce a rich mixture via the transfer channels which are remote from the outlet. Here air is already present ahead of the rich mixture in the transfer channels remote from the outlet. In this way, the mixture scavenging losses are intended to be reduced and therefore the exhaust-gas quality improved.
Supplied fuel-free gas, especially air, is not advantageous in all operating states for reducing scavenging losses. Thus, in the case of idle, an adequately rich ignition-ready mixture has to be provided in the combustion chamber; whereas, in high rpm ranges, a slightly lean mixture is advantageous. Furthermore, there is only a narrow window of time available for introducing the mixture at high engine speed which, in the case of advanced air, is further limited. In German patent publication 2,650,834, a throttle flap is suggested in the gas supplying air channel to adjust the quantity of the advanced air in the transfer channel remote from the outlet.
It is an object of the invention to improve a two-stroke engine to achieve a complete introduction of mixture at reduced scavenging losses independently of the charging principle at every operating point of the engine.
The two-stroke engine of the invention includes a two-stroke engine in a portable handheld work apparatus. The two-stroke engine includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; a mixture-preparation device for supplying an air/fuel mixture; an intake channel for conducting the air/fuel mixture into the crankcase; the cylinder having a discharge opening formed therein; a plurality of transfer channels connecting the crankcase to the combustion chamber; a first portion of the transfer channels being disposed on one side of a symmetry plane extending through the cylinder so as to partition the discharge outlet; a second portion of the transfer channels disposed on the other side of the symmetry plane; each of the transfer channels having a first end defining an entry window opening into the combustion chamber; the entry window being formed in the cylinder wall and being controlled by the piston as the piston moves in the cylinder; each of the transfer channels having a second end opening into the crankcase; a first plurality of air channels connected to corresponding ones of the first portion of transfer channels; a second plurality of air channels connected to corresponding ones of the second portion of the transfer channels; gas supply means for supplying a gas flow to each of the air channels; the first plurality of air channels being configured separately from each other so that the volumes of the gas flows thereto can be adjusted differently; the second plurality of air channels being configured separately from each other so that the volumes of the gas flows thereto can be adjusted differently; a check valve for connecting each of the air channels to the transfer channel corresponding thereto at a location between the first and second ends thereof so as to permit the gas flow to flow from the air channel into the transfer channel; and, a throttle device connected to the air channels for effecting the different adjustment of the volumes of the gas flows in the separately configured air channels.
Each transfer channel on a side of the symmetry plane is connected to a gas supplying air channel. The gas supplying channels are configured so that they are substantially separate from each other. In this way, it is possible to adjust the volumes of the gas flows in the air channels independently of each other so that the quantity and the distribution of the air, which is introduced ahead of the air/fuel mixture to reduce the mixture scavenging losses, can be metered so as to be adapted to the operating state of the engine.
For a two-stroke engine having (n) transfer channels (nxe2x89xa73), this makes possible the metering of the bypass air for reducing the mixture scavenging losses with this metering of the bypass air being adapted to an operating point of the engine. In this connection, the engine can be operated over a wide range in accordance with the stratified charge principle as well as in accordance with the principle of advanced air in the transfer channels close to the outlet as well as in the transfer channels remote from the outlet with an appropriate control of the gas-supplying air channels.
In the principle of advanced air, a volume of air is advanced into the transfer channel ahead of the air/fuel mixture thereby facilitating scavenging of the combustion chamber in advance of the next combustion. This reduces the amount of uncombusted fuel discharged to the atmosphere. More specifically, little of the air/fuel mixture which follows the advanced air is discharged because the discharge outlet is already almost closed or closed by the time that the air/fuel mixture reaches the combustion chamber behind the scavenging volume of air.
Advantageously, each air channel is assigned a preferably adjustable throttle. To reduce the need of components, a common adjustable throttle is assigned to the transfer pair of the overflow channels which lie approximately at the same elevation and are on opposite-lying sides of a plane of symmetry of the cylinder. The throttle is configured like an air throttle in a membrane carburetor and therefore has a throttle flap or cylinder which can be adjusted in a simple manner for controlling the volume flow. Here, it is advantageous that the throttle element of an air channel is position-dependently coupled to the throttle flap of the mixture-preparation device. This position-dependent coupling can be linear but can also be configured to be adapted to a curve, for example, progressively opening and can also include lost motion in order to make possible a time-dependent delayed opening of the air throttle element with reference to the carburetor throttle flap. This is achieved in a simple manner with the configuration of actuating arms or cam discs which connect the throttle flap levers of the various throttle flaps to each other. The coupling function is determined by the form and length of the actuating arms and cam discs. It is practical to be able to adjust the lengths of the actuating arms.